A typical thermal printer uses a ribbon with three or four donor patches (cyan, magenta, yellow and optionally clear protective layer laminate). Printing is typically done by a single print head that receives electrical signals while pressed against the donor ribbon and a receiver. Generally, a temporary laminate of donor ribbon and receiver is pulled thru the nip by a capstan roller at a controlled rate so as to minimize speed variations that would result in banding artifacts in the image. At the conclusion of the printing using color donor material from one donor patch of an image, the print head is raised, the donor ribbon is advanced to align the next donor patch with the receiver, and the receiver is moved to a start-of-printing position. Printing with patches and a single head requires relocating the receiver between each printing step and positioning the next color patch so that each color image plane of information can be transferred in register to the receiver. While effective for good image quality, such a mode of operation is wasteful for productivity since the rewind steps represent a portion of the total printing time.
In recent years there have been dramatic improvements in costs and thru-put of thermal printing of photos. However, there is still a need in the industry for printing faster, with little or no additional investment in printing hardware. Some of the recent improvements in print time are related to system optimization to reduce processing time. However, most of the recent improvements have come from decreasing the line time of the printer, from a modest 5 msec per line down to as little as a 1 msec per line. At short line times such as the latter, fundamental problems in the thermal imaging become major problems. Sticking of the donor to the receiver due to inadequate cooling of the donor materials and asymmetric thermal smear due to build up of heat in the print head are two issues that become significantly problematic.
It is also known in the art to provide 4-headed thermal printers. With this technology, each print head uses an individual supply of single-color donor ribbon, and printing is done in a continuous motion from start to finish. No rewinding of the receiver is required, and printing speed is generally very high because there is only one continuous printing. The ML500 printer sold by Eastman Kodak Company of Rochester, N.Y., U.S.A. is an example of such a 4-headed printer, and U.S. Pat. No. 5,440,328 describes a printer with three heads for a cyan, magenta and yellow (CMY) system. The use of a plurality of heads that print substantially simultaneously eliminates the need to rewind the paper and greatly improves productivity. In these systems, the receiver, usually in the form of a paper web is fed in a serial manner past the plurality of print heads.
FIG. 1 schematically illustrates a printer 8 according to the prior art having four print heads, four donor assemblies and a medium supply feeding receiver medium to each of the print head and donor assemblies. In the embodiment of FIG. 1, the four print heads 10, 12, 14 and 16 are positioned circumferentially about a large drum 18. Print heads 10, 12, 14, and 16 are provided with a donor ribbon 20, 22, 24 and 26, respectively. A receiver medium 28 is threaded around drum 18 so as to be between drum 18 and donor ribbons 20, 22, 24 and 26. Receiver medium 28 moves clockwise, as viewed in FIG. 1, first past print head 10 where a yellow color donor image is transferred to first receiver medium 28. A magenta color donor image is transferred to receiver medium 28 by print head 12, and a cyan color donor image is transferred to receiver medium 28 at print head 14. At print head 16, a protective lamination layer is transferred in a uniform manner. Receiver medium 28 having a completed print formed thereon is then stripped from drum 18.
The design of FIG. 1 simultaneously eliminated the need to rewind receiver medium 28 between the printing of color image planes and greatly improved productivity. However, 4-head thermal printers are inherently more expensive to build than are single head devices. They can also be more expensive in operation. Only one print head is energized at a time during print jobs containing only one 4-color image plane image. For print jobs that contain more than a single 4-color image plane image, any of the four print heads 10, 12, 14, and 16 can print simultaneously on separate receiver webs (not shown). In some embodiments of this type of printer a large receiver web leader is required to feed the receiver medium 28 through the system. This leads to waste, as the receiver medium leader must be trimmed away and discarded.
U.S. Pat. No. 5,841,460 describes a system that circulates a receiver sheet around a circular track to pass by a single print head many times so that overall cycle time can be reduced by eliminating the time required to rewind the receiver medium. Similarly, U.S. Patent Publication No. 2006/0171755 describes a printing system that attempts to achieve a similar result without a recirculating path by using two print heads to record image information on a receiver medium that is passed by the print heads in a reciprocal manner along a substantially flat path. In the '755 publication, the first print head is adapted to print when the medium moves in one direction along the reciprocating path, and the second print head records an image when the receiver medium moves along the other direction along the reciprocating path. Such a system provides reduced printing time as the time period required to rewind the receiver sheet between printing different color image planes is used at least in part for printing. It will be appreciated, however, that systems described in U.S. Pat. No. 5,841,460 and in U.S. Patent Publication No. 2006/0171755 only print on one receiver medium at any particular time and thus the overall cycle time for printing a plurality of images is simply a function of the number of images multiplied by the cycle time.
Other printers attempt to conserve printing time by using multiple print heads to simultaneously record images on different sides of the same receiver medium see for example, U.S. Patent Publication No. 2006/0158505 which describes such a printer. However, here too, the cycle time required to sequentially print each individual one of the three color image planes or the protective lamination layer is not reduced, instead a dual sided image is created within the same cycle time and thus the amount of time required to print a plurality of such images is simply a function of the number of images multiplied by the cycle time.
What is needed in the art therefore is a thermal dye diffusion printer that has a reduced overall cycle time for printing a plurality of images.